Re: [PATCH] fat: Provide option for setting timezone offset

[Jan Kara <jack@suse.cz>]

On Tue 13-11-12 01:50:48, OGAWA Hirofumi wrote:
> Jan Kara <jack@suse.cz> writes:
> 
> > On Mon 12-11-12 01:06:41, OGAWA Hirofumi wrote:
> >> Jan Kara <jack@suse.cz> writes:
> >> 
> >> > So far FAT either offsets time stamps by sys_tz.minuteswest or leaves them
> >> > as they are (when tz=UTC mount option is used). However in some cases it
> >> > is useful if one can specify time stamp offset on his own (e.g. when time
> >> > zone of the camera connected is different from time zone of the computer,
> >> > or when HW clock is in UTC and thus sys_tz.minuteswest == 0).
> >> >
> >> > So provide a mount option tz= which allows user to specify offset in minutes
> >> > that should be applied to time stamps on the filesystem.
> >> 
> >> What is in some cases? tz_minuteswest style timezone is known as it
> >> doesn't work. E.g. summer time.
> >   Yes, DST is one problem. Another problem (which is more annoying to the
> > user reporting this) is that he has HW clock set to UTC but system time is
> > in CET. Somewhat surprisingly (at least to me before I read the code) this
> > means sys_tz.minuteswest == 0. So when he connects say his camera, which
> > has time in CET, to the computer he sees timestamps off by one hour. With
> > tz= mount option he could mount the filesystem with tz=60 and be mostly
> > happy (modulo DST).
> >
> >> And tz= is reserved for true solution. E.g. load timezone database to
> >> kernel and use it for time conversion. So, if we really want this hack,
> >> it should be different option name.
> >   Yes, knowing about time zones in kernel is the only way to properly
> > handle fat timestamps in the presence of DST. But time zones are such a
> > mess (DST being determined by a law separately each year) I don't see this
> > happening - the annoyance by bad timestamps simply isn't big enough.
> >
> > If you feel strongly about reserving 'tz' mount option, I can rename the
> > mount option to something else... Would 'time_offset' be OK with you?
> 
> OK. It should be exclusive with "tz", and the option should be
> documented to
> 
> 	Documentation/filesystems/vfat.txt
> 
> as it doesn't work sometimes.
  That's a good idea. Thanks for reminding me. I'll send you an updated
patch in a moment.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR